Conflict detection and better error handling

Improvements in this release

• #14 Test suite (resolved #2)
• #15 Conflict detection (resolved #10)
• #16 Fix assembly registry bug (fixed #12 and resolved #13). This also greatly improved error messages when incorrectly configuring registries, both AssemblyEvenRegistry and other types.

Fix versioning

This will hopefully fix the version numbering both here and on NuGet.

Initial proof of concept

This is just to show that a project like this is possible, and to open up for the community. It has all the features you need to start using Event Sourcing, but it's still lacking many of the features you will expect from a mature event store, such as conflict detection and a thorough test suite.